The present invention is directed generally to improvements in television receivers, and particularly to a novel sawtooth generator for use in the vertical deflection portion of a television receiver.
Conventionally, television receivers include a cathode ray tube which develops a raster scan in response to vertical and horizontal deflection currents applied to a yoke. Those deflection currents are synchronized with incoming vertical and horizontal sync pulses received as part of a composite television signal.
In some receivers, a vertical oscillator is included for developing a sawtooth waveform which is applied to the yoke. U.S. Pat. No. 3,863,106 illustrates one such vertical oscillator. In other receivers, a vertical oscillator is not used. Instead, a countdown circuit is included which generates local clock pulses of twice the standard television line frequency. The clock pulses are counted and an internal vertical sync pulse is generated when a count indicative of the proper line rate/frame rate ratio (525 for NTSC systems) is reached. The internally generated sync pulse is applied to a sawtooth generator for developing a sawtooth waveform which is employed to control vertical deflection of the cathode ray tube. U.S. Pat. Nos. 3,691,297 and 3,916,102 disclose typical countdown systems of this type. An improved countdown system is disclosed in U.S. application Ser. No. 42,697, filed May 25, 1979, entitled VERTICAL SYNCHRONIZATION SYSTEM.
In television receivers employing a countdown circuit rather than a vertical oscillator, the internally generated vertical sync pulse is typically applied to an edge-triggered monostable multivibrator to generate a rectangular pulse of a predetermined duration. The latter pulse is then applied to a sawtooth waveform generator which provides vertical deflection of the cathode ray tube.
Although the use of a monostable driving a sawtooth waveform generator is satisfactory from a performance standpoint, it suffers from being somewhat expensive. For example, the monostable multivibrator usually employs a resistance-capacitance network to establish a predetermined time constant for the multivibrator. That time constant determines the duration of the pulse generated by the multivibrator.
In the sawtooth waveform generator, another capacitor is usually included for integrating the pulse provided by the multivibrator to develop a sawtooth waveform. Thus, the multivibrator-sawtooth generator combination requires two capacitors. When the vertical deflection circuitry is fabricated as an integrated circuit, the two capacitors are usually "discrete" and coupled to the integrated circuit by a pair of pin connections. The inclusion of two such pin connections obviously raises the cost of the integrated circuit. The cost of the two discrete capacitors raises the cost of the receiver even further. Hence, even though the multi-vibrator-sawtooth waveform generator performs satisfactorily in a receiver employing a vertical countdown system, its expense is a definite disadvantage.